1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which again conveys a sheet on which an image is formed to an image forming portion.
2. Description of the Related Art
In an image forming apparatus, sheets of various materials are fed to an image forming portion by a sheet conveying apparatus, and an image is formed on the sheet in the image forming portion and the sheet is discharged.
In the image forming apparatuses of recent years, to enhance the productivity, speed-up tendency and capacity-increasing tendency of image forming processing are accelerating, and a conveying speed of sheets also increasing. With the speed-up tendency of the sheet conveying operation, the conveying stability of sheet is more required.
Some image forming apparatuses again convey a sheet on which an image is formed to the image forming portion by a re-conveying portion, and forms a new image on the previously formed image. Some image forming apparatuses have a mode for forming images on front and back surfaces of a sheet (both-surface image forming mode, hereinafter). This technique is described in Japanese Patent Application Laid-open No. 2003-122061).
When images are formed on both surfaces of a sheet, the sheet is reversed at the re-conveying portion provided in the image forming apparatus, and images are formed on front and back surfaces of the sheet.
Conventionally, a sheet reverse portion which is one example of the re-conveying portion employs a switch back system. FIG. 19 shows a structure of the sheet reverse portion of the switch back system. When a sheet is to be reversed, a sheet S fed from the sheet feeding portion 112 passes through a vertically conveying path 120 and is conveyed downstream. The vertically conveying path 120 is provided at its downstream side with a registration roller apparatus 130. The registration roller apparatus 130 corrects final screw feeding of a sheet, synchronizes between an image writing operation in an image forming portion 140 and a sheet conveying operation.
The image forming portion 140 is provided at its downstream side with a fixing device 150. When a sheet passes through the fixing device 150, a toner image is fixed to a first surface of the sheet as a permanent image. When an image is to be formed on a second surface which is a back surface of the sheet, the sheet is conveyed to a sheet reverse portion 170 which also serves as an entrance to the duplex conveying path 180 by switching a conveying path switching means (not shown) provided downstream from the fixing device 150.
Next, the sheet is switched back by the sheet reverse portion 170, the sheet is conveyed to the duplex path 180. The sheet S which is reversed by such a conveying operation is conveyed to the image forming portion 140 through the vertically path 120, and an image is formed on the second surface of the sheet. After the image is formed on the second surface in this manner, the sheet S is conveyed to the fixing device 150, the second surface on which the images is fixed by the fixing device 150 is discharged out from a printer body 200 through the discharge path 160, and the sheet S is received by a discharge tray 161.
In the case of such a switch back system, if a large amount of sheets are conveyed at high speed, the probability that a paper jam occurs is increased. A location where a jam is prone to occur is an entrance portion of a sheet conveying path such as the arc vertically conveying path 120, and a junction constituting a sheet conveying path, and timing at which a jam is prone to occur is when a sheet moves toward a roller nip.
When a sheet is conveyed at high speed, especially when the sheet passes through an arc path, a sliding resistance between the sheet and a conveying guide is increased. If the sliding resistance is increased, a sheet may be folded, the sheet is charged with static electricity and the sheet is attracted, the sheet may slide on a guide plate, and a surface of the sheet on which an image is formed may be damaged.
When a sheet is conveyed at higher speed, if the sheet irrupts into the roller nip portion, when the sheet is soft and thin, the sheet does not easily enter the roller nip portion, and when the sheet is not nipped by the roller, the behavior of a tip end of the sheet becomes unstable due to an air resistance and a resistance of a guide plate.
In the case of the switch back system, in a reverse portion 170, the switch back of a sheet is carried out between other two sheets. Therefore, it is necessary to normally and reversely rotate the roller at high speed, sheets which are switched back in the path slide on each other, and the probability that a jam occurs due to slip of the roller is high.
That is, in the conventional image forming apparatus, a plurality of arc path portions exist and sheets are delivered at the duplex conveying portion and the reverse portion, and a jam is prone to occur at these portions. If the number of these portions is increased, the probability that a jam occurs is increased.
The duplex conveying portion and the reverse portion are constituted such that the conveying path is surrounded by a guide plate. Therefore, a sheet and the guide plate which receives high heat in the fixing device carry the heat to both-surface portion, and this increases the temperature in the apparatus, heat is accumulated in these portions, and this increases the temperature in the apparatus. If the temperature in the apparatus rises, the temperature of the image forming portion is increased by the sheet in the both-surface image formation, and appropriate images can not be formed.
To solve the problem caused when a sheet is conveyed by such a conventional switch back system, there is a technique in which a sheet is reversed by conveying the sheet while twisting the sheet using two belts. This technique is described in Japanese Patent Application Laid-open No. 2002-020000.
In the case of this structure, since a sheet is not delivered or the arc path portion does not exist unlike the roller conveyance, it is possible to prevent a jam from being generated. Since a guide plate which surrounds the conveying path does not exist, it is possible to prevent a temperature in the apparatus from rising.
In the case of the conventional image forming apparatus in which a sheet is again conveyed while twisting the sheet, the conveyance distance of a sheet is increased in order to twist the sheet and reliably turn the sheet over. As a result, the image forming apparatus is increased in size.
Further, if a sheet is turned over while moving the sheet at high speed, since the sheet is deviated in the twisting direction in some cases, the turning over and conveying speed of a sheet is limited. That is, when a sheet is conveyed while twisting the same, there is a problem that the image forming apparatus is increased in size and it is not possible to increase the conveying speed of a sheet.